The invention pertains to an interface between the channel units of an incumbent local exchange carrier and a competitive local exchange carrier. More particularly, the invention pertains to an interface for a competitive local exchange carrier interfacing to an incumbent local exchange carrier""s digital subscriber loop under the United States Telecommunications Act of 1996.
In accordance with the United States Telecommunications Act of 1996, incumbent local exchange carriers (ILECs) that, up to that time, had a monopoly on local telephone service in a given geographic area were required to provide to competitive local exchange carriers (CLECs) access to their connections to subscribers. Accordingly, CLECs could then offer competitive local telephone services to the original ILEC""s customers.
The purpose of the Telecommunications Act of 1996 was to foster competition for local telephone service by assuring that a CLEC could provide services to telephone subscribers with quality comparable to that provided by the ILEC. Accordingly, ILECs need to provide an interface between its central office terminals (COTS) and a CLEC""s remote terminal (RT). Despite the name, the CLEC""s remote terminal (RT) typically is located in the ILEC""s Central Office (CO). The CLEC then typically transports the communication channels via its own network facilities from its RT to its own office where it maintains its own telecommunications equipment, including switching fabric.
There are three common schemes in use today for interfacing between an ILEC""s COT and a CLEC""s RT. The first is an analog VF interconnection as illustrated in FIG. 1. In short, a twisted wire pair 12 is provided between the ILEC""s COT 14 and the CLEC""s RT 16 and the communication data is transmitted between the two terminals in analog form.
At least one goal of the Telecommunications Act of 1996 is to assure that a CLEC can provide the same grade of service (GoS) as the ILEC to telecommunication subscribers. However, many, if not most, ILEC telecommunications networks utilize digital loop carrier (DLC) systems 18 between their central office terminals, e.g., COT 14, and remote terminals, e.g., RT 20, located closer to the subscribers"" telephone 22. Typically, remote terminal 20 is located within about two miles of the particular subscriber and the signaling between the central office terminal 14 and the remote terminal 20 is digital, termed a digital loop carrier or DLC 18. The DLC portion 18 is a 4-wire portion comprising two twisted wire pairs 18a and 18b, one of which carries downstream signals and the other of which carries signals upstream. For those subscribers that do not have digital service, a CODEC 24 in the RT interfaces with the subscriber""s telephone equipment 22 and converts the signals between analog and digital. In the U.S., for example, in the digital loop carrier portion 18 of the network, voice (and data) is sampled at the rate of eight KHz and digitized into eight bit samples, yielding a 64 Kbps (8 KHz times 8 bits) data rate. A digitized voice channel (64 Kbps) is called a DSO. In many if not most digital loop carrier systems, 24 digitized voice channels (DSOS) are time division multiplexed into one signal called a DS1 signal.
When an ILEC provides local area telephone services to its subscribers using digital loop carrier systems between its RTs 20 and COTs 14, the signaling between terminals is all digital. However, when a CLEC provides the local area service and interconnects to the ILECs COT via an analog VF interconnection, a digital to analog conversion must be performed in the ILEC""s channel unit before the data can be transmitted over the analog interface. Specifically, a channel unit 26 in the COT 14 must convert signals that are to be transmitted to the CLEC RT 16 over the interface 12 from digital to analog and convert signals received from the CLEC RT 16 from analog to digital.
The CLEC""s switching fabric 29 typically is located in the CLEC""s central office 31 at a remote location from the ILEC""s central office 27. Accordingly, the CLEC""s RT interfaces with the CLEC""s Central Office via another link 33, which is commonly a digital carrier link, such as another DLC. Therefore, the CLEC""s RT 16 must convert signals that are to be transmitted to the ILEC""s COT 16 over the interface 12 from digital to analog and convert signals received from the ILEC""s COT 16 from analog to digital. Accordingly, the ILEC""s COT channel unit 26 and the CLEC""s RT channel unit 28 both contain CODECs 25 for converting upstream channels through the interface 12 from analog to digital and converting downstream channels from digital to analog.
Further, the interface between the ILEC COT 14 and the CLEC RT 16 is a single twisted wire pair 12 in which upstream and downstream data are carried on the same wire pair, whereas the digital loop carrier portion of the system is a four wire system with the upstream and downstream signals being transmitted on separate twisted wire pairs. Therefore, each channel unit 14, 16 also includes a hybrid circuit 23 for converting between 2-wire (the interface side) and 4-wire (the DLC) transmission modes.
Specifically, there are at least three aspects of this link which degrade the service when signals are exchanged between the ILEC and the CLEC. First, there is an increase in background (or quantization) noise. Each time an analog to digital conversion occurs, quantizing noise is introduced because of the finite granularity of the digital code representing each sample of the analog signal. When the ILEC provides service, there is only one analog/digital conversion (at the ILEC""s RT 20 or the ILEC""s digital switch). However, when the CLEC provides analog-to-digital service, there are two analog-to-digital conversions namely, at the ILEC""s RT and at the CLEC""s RT 16. Accordingly, the background noise when service is provided by a CLEC can be twice that of when service is provided directly by the ILEC.
Secondly, additional noise is introduced by the repeated use of robbed bit signaling (RBS). Robbed bit signaling allows the digital voice or data bit stream to carry signal states necessary for supervision of telephone circuits. A robbed bit is sent in the least significant bit (LSB) position of every 6th voice/data sample. In essence, one out of every 48 bits transmitted is xe2x80x9crobbedxe2x80x9d for supervisory purposes. When a CLEC provides service, the signals will contain an extra set of robbed bits since the CLEC will add its own robbed bit in its own network while the signal will still contain the robbed bit from the ILEC""s network.
Thirdly, the back to back channel unit configuration used for interconnecting the CLEC and ILEC networks introduces two extra sources of echo. Specifically, when the interconnection between the ILEC and the CLEC is two wire, there must be a hybrid circuit in each channel unit to convert between 2- and 4-wire, thus introducing two more sources of echo in the link. The extra echoes can be avoided by using a 4-wire rather than a 2-wire interface.
A second option for the interface between the ILEC""s COT and the CLEC""s RT is a digital cross-connect system (DCS) I/O. A DCS I/O has a plurality, e.g., 24, of DS1 interfaces and a time slot interchange matrix that can be used to cross connect DSO channels. However, a DCS I/O interface is expensive. Further, it provides only DS1 interfaces. Thus, bandwidth is wasted when less than 24 channels are being interconnected.
Another existing interface is the DS1 interconnection. However, like the DCS I/O solution, this type of interface also has 24 channels and is expensive. Accordingly, bandwidth is wasted when less than 24 channels are being interconnected.
Accordingly, it is an object of the present invention to provide an improved interface between the channel units of an ILEC and a CLEC.
In accordance with the invention, an interface between an incumbent local exchange carrier""s central office terminal and a competitive local exchange carrier""s remote terminal is provided by a modified Integrated Services Digital Network (ISDN) type interconnection. Preferably a modified Basic Rate ISDN U-Interface (BRI-U) in accordance with ANSI standard T1.601 is preferred. Such a connection provides two data clear channels of 64 Kbps each and two utility channels. The utility channels comprise a signaling channel of 16 Kbps and a framing/overhead channel of 16 Kbps (comprising 12 Kbps of ISDN framing structure and 4 Kbps of overhead). The total data rate of the BRI-U is 148 Kbps. The total bit rate is 160 Kbps. The 148 Kbps data signal can be carried over a single wire pair using a 2B1Q line code. The interface may be used to interconnect one or two 64 Kbps channels. The channel units may be designed to remove the robbed bits from the data channels and place them in either the signaling channel or the overhead channel. Also, a superframe (or extended superframe) synchronization signal can be transmitted in the 4 Kbps overhead portion of the framing/overhead channel so that the receiving channel unit can use that information to compensate for offset between its digital carrier frames and the digital carrier frames of the transmitting channel unit. Specifically, (1) the bit weights in the framing structures of the two networks should match so that the data samples are properly designated and (2) the robbed bit signaling should fall in the same bit positions for both networks so as to avoid double robbed bit signaling.